This counts LTRs this device received that it cannot tie back to a linktrace session it remembers starting, the path-trace analog of the unresolved-LBR counter. It is most often seen when a linktrace's local transaction record has already expired, because the trace took longer than this device's timeout, before a distant hop's reply finally arrives. Seeing this counter increment specifically on traces aimed at targets many hops away, while short local traces never trigger it, is consistent with round-trip time simply exceeding this device's session timeout on long paths.